The present invention relates to exhaust lines for two-stroke internal-combustion engines, comprising at least one catalyst.
The invention may be particularly applied to outboard engines, for which size constraints are very severe.
According to the prior art such as shown for example in FIG. 1, an optimized exhaust line of a two-stroke engine may consist of a divergent zone 1a positioned downstream from port 2 or from the exhaust ports, followed by a constant section zone 2a and possibly by a convergent zone 3a.
This geometry profits from the acoustic effects of the pressure waves and thereby improves the filling, and therefore the efficiency of two-stroke engines, over the widest possible speed range. More precisely, this type of exhaust line is designed, on the one hand, to favour the extraction of the waste gases during the scavenging stroke and, on the other hand, to avoid fresh gas losses at the exhaust, at the end of the scavenging stroke. To achieve these two functions, the exhaust line must allow formation of a depression wave at the level of the exhaust port during the scavenging stroke around the bottom dead center, and then formation of a compression wave at the level of this port at the end of the scavenging stroke, before the exhaust line closes.
The geometry defined above allows these characteristics to be achieved: in fact, the depression wave is obtained through the divergent zone 1a while the compression wave may be obtained by means of the convergent part 3a of such an exhaust line.
It should however be noted that, in multicylinder engines, the compression wave may be obtained by using a lay-out such as that shown in FIG. 2 between the exhaust manifolds of the various cylinders. This lay-out will allow the angular offset between the various cylinders to be used since the compression wave generated through the opening of the exhaust port of a cylinder, which reaches the level of the exhaust port(s) of another cylinder shortly before it closes, that is at the end of the scavenging thereof, enables compression to be achieved in this cylinder. Of course, the opening times of the ports and the lengths of the individual manifolds must be calculated appropriately, according to the number of cylinders of the engine.
In FIG. 2 relating to a multicylinder engine according to the prior art, the divergent part 1a appears in dashed lines because it might not exist. In this case, only a compression wave is created, as shown above. The depression wave not being present, the running and in particular the power of the engine is decreased in relation to an engine fitted with an exhaust provided with a divergent.
FIGS. 3A, 3B and 3C illustrate various examples of exhaust lines 30 of well-known outboard engines. The characteristic common to these exhaust lines is their compactness: in fact, the divergent and the convergent, when there is one, are "imbricated" in each other. FIG. 3A shows the case of an exhaust line 30 of a single-cylinder marine engine comprising for example a divergent zone 31 through which the exhaust gases run before spreading through convergent zone 32. The gases escape through the lower part 33 of the exhaust system, that is into the water.
FIG. 3B is different from FIG. 3A in that there are two cylinders instead of only one. A divergent zone and a convergent zone are present.
Finally, FIG. 3C relates to a three-cylinder engine comprising no convergent, the connection and the angular offset between the three cylinders playing the same part.
If one wishes to provide such exhaust lines with at least one catalyst for extracting certain pollutants from the exhaust gases, various locations may be considered.
References 35 and 35' in FIGS. 3A to 3C show possible locations for these catalysts. However, these solutions have the following drawbacks:
The acoustic effects are particularly reduced, or even totally inhibited, by the addition of such catalysts, notably because of the pressure drops they generate in the exhaust line and of the change in the propagation of the wave at the level of the catalyst itself (the latter generally comprises multiple channels of very small section). Engine performances may therefore be markedly reduced, so much so that two-stroke engines lose their characteristics related to a high specific power.
Furthermore, addition of a catalyst implies a temperature rise in the exhaust line, both at the level of the catalyst and downstream therefrom. This heating is related to the amount of pollutants emitted. It may generate catalyst overheating problems, which has a negative influence on its efficiency, its service life, and may even lead to its destruction. Besides, the environment of the exhaust manifold may also be damaged by such an overheating.
Cooling of the wall is therefore provided to remedy the overheating of the exhaust manifold and/or of the cylinder block. However, this cooling system is not sufficient when a catalyst is installed in the exhaust manifold.
In the particular case of an outboard engine (See FIGS. 3A to 3C), water, which may be sea water, is present in the exhaust manifold 33, both for cooling and sound insulation. It is therefore not advisable, in this case, to position a catalyst such as 35 close to the exhaust outlet because water might corrode this catalyst 35.
To avoid this drawback, the catalyst should be placed far from the exhaust outlet, that is close to the engine (See 35' in FIGS. 3A to 3C). But, of course, the temperature of the gases being very high at the combustion chamber outlet, a catalyst (such as 35') positioned directly at this level would be destroyed and burned even faster.
This latter solution is therefore not conceivable.
Besides, the prior art has already disclosed means for avoiding overheating of filters and/or of catalysts installed in exhaust lines.
Thus, document DE-3,406,968 provides two different pipes, one cooled, the other not cooled, which join just upstream from the catalyst and are associated with flow control systems so as to modulate the temperature of the gases running through the catalyst.
French patent FR-A-2,608,677 recommends the use of two manifolds opening at two different levels in the combustion chamber, associated with a distribution means allowing the flow of gases coming from each manifold and crossing a catalyst located downstream from the manifolds to be controlled. The problem at the origin of this invention is related to the time necessary for operating the catalyst during cold startings. The object of this invention is to decrease this time, called "initiation" time, during which the catalyst has no influence on the pollutants contained in the exhaust gases.